The invention relates generally to the field of marine geophysical surveying. More particularly, at least in some embodiments, the invention relates to devices and methods for controlling the depth profile of marine geophysical sensor streamers as they are towed in a body of water.
Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, include towing an energy source at a selected depth in a body of water. One or more geophysical sensor streamers also may be towed in the water at selected depths. The streamers are essentially long cables having geophysical sensors disposed thereon at spaced apart locations. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom. Reflected energy from interfaces, generally at the boundaries between layers of rock formations, is returned toward the surface and is detected by the sensors on the one or more streamers. The detected energy is used to infer certain properties of the subsurface rock formations, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
For certain types of surveying, it is important that a streamer is maintained as closely as possible to a selected depth profile in the water. For example, electromagnetic surveying using a towed streamer is currently limited by the noise originating from towing the streamer through the water. Towing noise may be reduced by maintaining the streamer at a substantially constant depth, thereby keeping the streamer as level as possible. Devices commonly used to regulate streamer depth include lateral force and depth (LFD) control devices. For example, U.S. Pat. No. 6,144,342 issued to Bertheas et al. describes a structure for LFD control devices and a method for controlling the navigation of a towed seismic streamer using “birds” affixable to the exterior of the streamer. The birds are equipped with variable-incidence wings and are rotatably fixed onto the streamer. Through a differential action, the wings allow the birds to be turned about the longitudinal axis of the streamer so that a hydrodynamic force oriented in any given direction about the longitudinal axis of the streamer is obtained. Power and control signals may be transmitted between the streamer and the bird by rotary transformers. (In some applications, birds may utilize localized battery power.) The bird is fixed to the streamer by a bore closed by a cover. The bird can be detached automatically as the streamer is raised so that the streamer can be wound freely onto a drum. The disclosed method purportedly allows the full control of the deformation, immersion depth, and heading of the streamer.
Often, such LFD control devices are used in conjunction with sensors, such as pressure sensors, capable of generating a signal related to depth. The sensors may be positioned along the streamer. Typical pressure sensors used in geophysical surveying can be calibrated to a precision of about 0.1 percent of the full scale range of the sensor. While a streamer using only pressure sensors for depth measurement can be navigated laterally in the water to a precision of about 1 meter, at a water depth of about 1000 meters and greater, the possible error in navigation of the streamer in the vertical plane becomes proportionately larger.
Some survey conditions necessitate towing of streamers with various non-level depth profiles. For example, it may be optimal to tow a streamer with a constant gradient to optimally follow the survey area bathymetry, thereby optimizing the signal-to-noise ratio in the received signal.
What is needed is a system that can assist in navigation of a geophysical sensor streamer in the vertical plane at relatively great water depth.